In oil well pumps, the walking beam at the well head is connected by a sucker rod to the pump at the bottom of the well. The sucker rod is usually of great length, often in excess of 10,000 feet and must be of high tensile strength to support even its own weight. Furthermore, the rod is immersed in the crude oil of the well which is highly corrosive. The rod for a well is made up of many individual rod-lengths which are joined end-to-end, each rod-length being about twenty-five feet long. The joint between the rod-length is typically in the form of a threaded fitting.
One widely used structure of a rod-length is commonly known as a "three-piece rod". In this structure, the rod is threaded at both ends and a rod-end fitting is threadedly engaged with each end of the rod to form a three-piece assembly. The free end of the rod-end fitting is adapted for connection to another rod-end fitting. One form of three-piece rod assembly uses, on each end of the rod, a fitting which has a threaded male pin on the free end, such fitting being known as the "pin end". With this, the three-piece rods are joined to each other by a conventional threaded coupling which is screwed onto adjoining pin ends. In another form of three-piece rod, one end of the rod is fitted with a pin end and the other end has a fitting with a threaded socket at its free end, the latter being called a "box-end". With this, the rods are joined together by screwing the pin end of one into the box end of the other.
In the prior art, sucker rods, especially the three-piece rods, experience breakage too frequently in the region of the threaded connectors. In use, the rod is subjected to extremely high tensile stress, and also to bending stress due to changes in direction of the well casing. In particular, stress concentration tends to occur in the vicinity of the juncture of the threaded and unthreaded portion of the rod. Such breakage leads to significant amounts of down-time of the pumping apparatus while the rod is removed from the well shaft and replaced.
A general object of this invention is to provide an improved three-piece sucker rod assembly that overcomes the certain disadvantages of the prior art.